This invention relates generally to rolling bearings, and more particularly to rolling bearings which have corrosion resistance and fracture toughness and to methods for making the bearings.
Anti-friction track rolling bearings are commonly used in a variety of machines and, in particular, in aircraft where the bearings are subject to high loads, and are exposed to corrosive environments. The high loads increase the surface pressure applied to the bearing elements causing metal fatigue leading to failure of the bearing. A unique feature of rolling bearings is that their useful life is not determined by wear but by metal fatigue of one of the elements, e.g., the inner race, the outer race or the rolling elements, under the repeated stresses of normal use. Fatigue failure, which occurs as a progressive flaking or pitting of the surfaces of the races and rolling elements, is accepted as the basic reason for the end of the useful life of such a bearing. In particular, the outer race experiences cyclic flexing which causes tensile stressing leading to fracture. The bearing elements need fracture toughness, specifically internal compressive stresses to counter applied tensile stress.
Because the load on a bearing determines the severity of the stress in the surfaces of the races and the rolling elements, if the load is increased the life of the bearing is shortened. This is usually expressed by the relationship in that the life of the bearing is inversely proportional to the load cubed. Thus, doubling the load will reduce the life of the bearing by a factor of 8. The life of a bearing under a given load will therefore be a certain number of revolutions. When a bearing is intended for use under heavy loads, such as in aircraft, it is desirable that the bearing have hardened surfaces to accommodate the added stresses.
To increase the strength of bearing elements, it is common to heat the steel alloy to carburize the alloy surface. Carburizing increases the hardness of the bearing surfaces to extend the life of the bearing, however, carburizing increases the carbon content of the steel which lowers corrosion resistance. AISI 52100 steel is widely used in these bearings because of excellent fatigue properties but does not have corrosion resistance. The material which has traditionally been used to improve corrosion resistance, AISI 440C stainless steel, does not have the fatigue properties comparable to AISI 52100 steel for fracture toughness. Thus, it is desirable to provide rolling bearings having increased fatigue life and corrosion resistance.
The foregoing illustrates limitations known to exist in present devices and methods. Thus, it is apparent that it would be advantageous to provide an alternative directed to overcoming one or more of the limitations set forth above. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.